1. Field of the Invention
The present invention relates to a power supply apparatus for use, for example, in a vehicle to drive a motor for making the vehicle run, or in an electric appliance to drive a load. More particularly, the present invention relates to a power supply apparatus incorporating a temperature detection circuit for detecting the temperature of a battery.
2. Description of Related Art
A vehicle-driving battery for supplying electric power to a motor for driving a vehicle yields a high output voltage thanks to a large number of battery cells, secondary cells, connected in series. This permits a motor to yield an output high enough to make an automobile run. In a power supply apparatus incorporating such a vehicle-driving battery, when the battery is made to discharge a large current to make a motor sharply accelerate an automobile, or when the battery is charged nearly to the fully-charged state with a large current while an automobile is running down with regenerative braking applied, or when the outside air temperature and thus the ambient temperature is abnormally high, or under other conditions, the temperatures of the battery cells become considerably high.
Moreover, a power supply apparatus for a vehicle has a very large number of battery cells, as many as 100 or more of them, connected in series to yield an output voltage as high as 200 V to 400 V. When a large number of battery cells connected in series are charged and discharged, an equal current flows through all the battery cells. However, not all the battery cells deteriorate evenly; that is, only one or more particular battery cells may deteriorate. Variations inevitable in the manufacturing process, differences in external/ambient factors such as the use temperature, etc. cause the individual battery cells to deteriorate unevenly. If any battery cell deteriorates and the full capacity to which it can be charged becomes substantially smaller, charging and discharging that battery cell in the same way as the other battery cells cause it to be overcharged or overdischarged. This makes the cell temperature higher, further promoting the deterioration.
Typically used as battery cells for vehicle-driving batteries are nickel hydride cells and lithium ion secondary cells. Both types of secondary cell exhibit poorer characteristics at higher temperatures, and, at still higher temperatures, exhibit thermal runaway and quickly deteriorate. To prevent this, in a power supply apparatus for a vehicle, the temperatures of individual battery cells are detected. If the temperature of any battery cell becomes, for example, higher than 55° C., the contactor connected to the output side of the vehicle-driving battery is turned off to cut the current and thereby prevent the detrimental effects of raised temperature.
To achieve this, in a conventional power supply apparatus, PTC (positive temperature coefficient) sensors serving as temperature sensors are fixed to and kept in contact with the surface of individual battery cells. This type of power supply apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. H10-270094 (hereinafter referred to as Patent Publication 1). In the power supply apparatus disclosed in Patent Publication 1, a large number of PTC sensors fixed to individual battery cells are connected in series, and the total resistance of the serially connected PTC sensors is measured. When the temperature of a PTC sensor becomes higher than a set temperature, its electrical resistance abruptly becomes higher. Thus, when the temperature of any of the battery cells becomes higher than the set temperature, the electrical resistance of the serial circuit composed of the PTC sensors becomes higher. Hence, by measuring the electrical resistance of the serial circuit, it is possible to detect that there is a battery cell whose temperature is higher than the set temperature.
However, in a case where a large number of PTC sensors are used, it is impossible to accurately detect the temperatures of the individual battery cells. This is because it is difficult to give all the PTC sensors equal characteristics. For example, with a circuit for detecting the temperatures of individual battery cells by the use of a large number of PTC sensors whose set temperature, i.e., the temperature over which their electrical resistance abruptly increases, includes an error of up to 10%, it is impossible to accurately detect, for all the battery cells, whether or not the temperatures of the individual battery cells are higher than the set temperature. Moreover, in a case where, as disclosed in Patent Publication 1 mentioned above, a large number of PTC sensors are connected in series and their total electrical resistance is detected, the larger the number of PTC sensors serially connected, the more difficult it is to accurately detect the temperatures of the battery cells. This is because, the larger the number of PTC sensors serially connected, the smaller the proportion of the rise in their total electrical resistance that occurs when the electrical resistance of any PTC sensor increases.